Degaussing circuit with bucking current source for reducing current in degaussing coil to zero

ABSTRACT

A degaussing circuit used in a color television receiver includes a positive temperature coefficient resistor connected in series with the degassing coil, and in addition includes a circuit coupled with the filament winding of the receiver for applying a current through the coil in phase opposition to the current through the positive temperature coefficient resistor. The opposition current is of a magnitude equal to the residual current through the positive temperature coefficient resistor to reduce the current in the degaussing coil to zero after the degaussing operation is complete.

United States Patent Hansen et al.

[541 DEGAUSSING CIRCUIT WITH BUCKING CURRENT SOURCE FOR REDUCING CURRENT IN DEGAUSSING COIL TO ZERO [72] Inventors: Robert B. Hansen, Arlington Heights; William H. Slavik, Oak Lawn, both of I11. [73] Assignee:

Motorola, Inc., Franklin Park, ill.

[22] Filed: May 6, 1970 [21] Appl. No.: 34,985

[52] U.S. Cl ..3l5/8,3l7/157.5 [51] Int. Cl. H01j 31/20, HOlf 13/00 [58] Field of Search ..315/8 [56] References Cited UNITED STATES PATENTS 3,492,543 1/1970 Muranaga ..315/8 x [is] 3,657,729 [4 1 Apr. 18, 1972 Primary ExaminerRobert Segal Attorney-Mueller & Aichele 57 ABSTRACT A degaussing circuit used in a color television receiver includes a positive temperature coefficient resistor connected in series with the degassing coil, and in addition includes a circuit coupled with the filament winding of the receiver for applying a current through the coil in phase opposition to the current through the positive temperature coefiicient resistor. The opposition current is of a magnitude equal to the residual current through the positive temperature coefficient resistor to reduce the current in the degaussing coil to zero after the degaussing operation is complete.

3 Claims, 1 Drawing Figure COLOR TELEVISION RECEIVER Patented April 18, 1972 3,657,729

COLOR TELEVISION RECEIVER INVENTORS ROBERT E HANSEN WILLIAM H. SLAVIK.

MX/M

ATTYS.

BACKGROUND OF THE INVENTION The three-gun color television picture tube has a shadow mask consisting of a large number of apertures to properly restrict the beam from each gun into its associated phosphor dots on the picture tube screen. The shadow mask, along with a frame to mount the mask, and additional metal structure within the tube may become undesirably magnetized by external magnetic fields. Unless corrected, such magnetization may degrade color reproduction by the television receiver.

Demagnetization or degaussing may be accomplished by positioning a coil adjacent to the picture tube and providing circuitry for causing a decaying alternating current to flow through the coil with a high initial amplitude. Automatic degaussing circuits for accomplishing this result are generally employed in color television receivers, so that each time the television receiver is turned on, demagnetizing current is applied to the degaussing coils. To produce the steadily decreasing alternating current, the degaussing coils are generally supplied from the alternating voltage source through a positive temperature coefficient supply resistance, the resistance of which increases in the loaded condition. After a predetermined period of time during which the degaussing operation takes place, the current through the positive temperature coefficient resistor and the coil for the greatest resistance value of the resistor produces a small remanence field which may be disturbing. As a consequence, it has been the practice to provide an additional switching circuit which switches off the degaussing current in the coil by an automatically operated switch when the amplitude of the degaussing current in the coil is sufiiciently small.

Other systems attempting to eliminate the necessity for this extra switch, which is an expensive component, have provided a negative temperature coefficient resistor in parallel with the degaussing coil, with the two resistors being thermally connected. Such a circuit results in an operation approaching that of the circuits using a switch to switch off the current through the degaussing coil when the minimum value is reached, but the operation still is not effective to totally eliminate the residual current flowing through the degaussingcoil. Even small residual currents may cause sufiicient purity shift at the difference frequency between the vertical frequency and the line frequency to produce a visible, unwanted degradation of the image produced on the cathode ray tube screen.

Thus, it is desirable to provide a means of eliminating this small residual current through the degaussing coil without the necessity of the additional, relatively expensive switch which -has been employed.

SUMMARY IOFITHE lNVEN'l'lON Accordingly, it is an object of this invention to provide an improved neutralizing circuit for demagnetizing a body of magnetizable material.

It is another object of this invention to reduce the residual current in a degaussing coil to near zero after a predetermined period of time following the application of an AC potential to the coil.

It is an additional object of this invention to reduce the residual current through a degaussing coil, coupled in series with a positive temperature coefficient resistor, to near zero by the application of an opposing AC current substantially equal to the residual current through the positive temperature coefficient resistor.

In accordance with a preferred embodiment of this invention, a neutralizing circuit for a body of magnetizable material includes a neutralizing coil adapted to be placed adjacent to the magnetizable body and coupled with resistance means having a coefficient of temperature to provide a time varying AC current from a maximum value to a minimum value through the coil within a predetermined period of time.

Further means applies an AC current to the coil in opposition to the current flowing through the coil as a result of the final resistance of the temperature coefficient resistor to substantially cancel out any residual current flowing through the coil.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of the drawing is a schematic diagram, partially in block form, of a preferred embodiment of the invention.

DETAILED DESCRlPTlON Referring now to the drawing, there is shown a color television receiver 10 which processes television signals picked up by an antenna 12 and converts them into color information for the multiple cathodes 14 of a three-gun color cathode ray tube 16. Power for operating the receiver 10 is obtained from a suitable source of AC voltage 22, which may be supplied from a conventional wall outlet. A switch 24 serves as the on-off switch for the receiver and is accessible to the viewer. A terminal 26 of the switch is connected to the primary winding of a transformer 30, so that closure of the switch contact 24 on the terminal 26 supplies alternating current potential across the winding 28, with the junction of the winding 28 and the AC source 22 being connected to ground.

A secondary winding 32 on the transformer 30 is connected to a bridge rectifier circuit 34 which, in combination with a load capacitor 36 and an inductor 38, supplies a DC voltage on a lead 18 as the operating potential for the color television receiver 10. Generally, of course, more than one DC voltage would be developed by the power supply; but for the sake of convenience and simplicity only one is shown here. A filament energizing circuit for the filaments 50 of the cathode ray tube 16 is provided by a second secondary winding 48 on the transformer 30, with the end temtinals of the secondary winding 48 being connected in parallel to each of the three filaments 50 associated with the cathodes 14.

With the on-off switch 24 in the position shown, no AC current is applied to the primary winding 28 of the transformer 30. Therefore, no operating potential is applied to the receiver 10, nor is any filament voltage developed across the winding 48. Closure of the switch 24 on the terminal 26 then applies current through the primary winding 28 of the transformer 30, supplying filament current to the filaments 50 and DC operating current to the color television receiver 10.

The cathode ray tube 16 has a shadow mask 52 with a number of apertures for aligning the electron beams from the multiple cathodes 14 with the associated phosphor dots on a screen 54. A frame 56 extends circumferentially about the inner face of the tube 16 and, as shown, is used to mount the shadow mask 52. The frame, mask, and additional metal structure within the cathode ray tube 16 is susceptible to becoming magnetized by the earths magnetic field or other stray magnetic fields. In order to demagnetize the metal structure, a decaying alternating current is required. The peak value of the current must be at least high enough to cause momentary magnetization exceeding that which may exist in the metal structure in the cathode ray tube 16. Although a single cycle in theory will cause complete demagnetization, if the current is not shut off precisly when it passes through zero, there will be some residual magnetism left or added to what is already present in the metal structure. Allowing the alternating current to decay and removing the current when it reaches a 56, the coils may be divided into several fractional coils which substantially surround the metal structure within the cathode ray tube 16.

As is well known, when the receiver is first turned on, the positive temperature coefficient resistor 64 is relatively cool and has a relatively low resistance; so that a substantially large AC current is permitted to flow through the degaussing coils 62 and 63. Within a short period of time, selected to be less than the time required to commence producing images on the screen 54 of the cathode ray tube 16, the current flowing through the positive temperature coefficient resistor 64 and the degaussing or neutralizing coils 62 and 63 decays to a minimum value of current,,determined by the characteristics of the resistor 64. Ideally this current should decay to zero, but a small residual current continues to flow through the resistor 64 after completion of the degaussing or neutralizing operation. Although this residual current is near zero, it still may be sufficient to provide a purity shift at the difference frequency between the vertical frequency and the line frequency of operation of the television receiver.

As a consequence, it is desirable to eliminate this current. This is accomplished by connecting a resistor 71 between a tap 70 on the secondary winding 48'and the junction between the positive temperature coefficient resistor 64 and the coil 62. The polarity of the signals induced in the secondary winding 48 by the primary winding'28 of the transformer 30 is such that the current flowing through the resistor 71 to the coils 62 and 63 is in opposition to the current flowing through the positive temperature coefficient resistor 64 to the coils 62 and 63. The position of the tap 70 and the value of the resistor 71 are chosen to be such as to produce a current in the coils 62 and 63 which is equal to the small residual current flowing through the resistor 64 upon stabilization of the current through the resistor 64. Since the current flowing through the resistor 71 is in opposition to the current flowing through the resistor 64,

this added current cancels out the residual current produced through the resistor 64 in the coils 62 and 63 to decrease the residual current in the coils 62 and 63 to near zero.

It is to be noted that the bucking or cancelling current provided by the winding 48 and the resistor 71 flows throughout the degaussing interval; but since this current is of relatively small magnitude, it is swamped out by the large initial current flowing through the resistor 64 and has substantially no affect on the operation of the circuit. By the use of the out-of-phase current provided from the secondary winding 48 and the resistor 71, it is possible to design the circuit to provide an initially greater peak value, thereby resulting in an improved degaussing action in the degaussing coils 62 and 63, since the final residual current through the positive temperature coefficient resistor 64 is cancelled out by the proper selection of the value of the resistor 71 and the location of the tap 70 on the secondary winding 48.

Although the foregoing description has been made in conjunction with a positive temperature coefficient resistor connected in series with the degaussing coils 62 and 63 to provide the degaussing operation, it is apparent that a negative temperature coefi'lcient resistor also could be employed connected in parallel with the degaussing coils 62 and 63, if desired. The opposing current obtained from the secondary winding 48 would be utilized in the same manner as described above in conjunction with the use of a positive temperature coefficient resistor in the circuit.

We claim:

1. In a color television receiver having a cathode ray tube with filament means and structure subject to becoming magnetized, switching means for coupling a source of AC potential to the receiver, a degaussing circuit for demagnetizing said structure subject to becoming magnetized including in combination: 1 coil means having first and second ends and located adjacent the stmcture subject to becoming magnetized;

positive temperature coefficient resistor means coupled at a first terminal with the first end of said coil means and connected in series with said C011 means and the switching predetermined maximum value sufficient to demagnetize the structure subject to becoming magnetized to a predetermined minimum value; transformer means having a primary winding and at least one secondary winding, the primary winding being coupled in series with the switching means;

second resistance means coupled between the first tenninal and the secondary winding of said transformer means; and

the secondary winding of said transformer means having a tenninal connected in common with one end of the primary winding and the second end of said coil means, AC current developed in the secondary winding of said transformer means being in phase opposition to the AC current flowing through said positive temperature coefficient resistor means and said coil means and having a magnitude substantially equal to said minimum value, thereby to substantially cancel current through said coil means when the current through said positive temperature coefficient resistor means attains said predetermined minimum value.

2. The combination according to claim 1 wherein the terminal of the secondary winding is connected to a point of reference potential.

3. The combination according to claim 1 wherein the secondary winding of said transformer means is further coupled with the filament means of the cathode ray tube to provide filament current thereto. 

1. In a color television receiver having a cathode ray tube with filament means and structure subject to becoming magnetized, switching means for coupling a source of AC potential to the receiver, a degaussing circuit for demagnetizing said structure subject to becoming magnetized including in combination: coil means having first and second ends and located adjacent the structure subject to becoming magnetized; positive temperature coefficient resistor means coupled at a first terminal with the first end of said coil means and connected in series with said coil means and the switching means, said positive temperature coefficient resistor means being responsive to AC potential to develop a current through said coil means which decays from an initial predetermined maximum value sufficient to demagnetize the structure subject to becoming magnetized to a predetermined minimum value; transformer means having a primary winding and at least one secondary winding, the primary winding being coupled in series with the switching means; second resistance means coupled between the first terminal and the secondary winding of said transformer means; and the secondary winding of said transformer means having a terminal connected in common with one end of the primary winding and the second end of said coil means, AC current developed in the secondary winding of said transformer means being in phase opposition to the AC current flowing through said positive temperature coefficient resistor means and said coil means and having a magnitude substantially equal to said minimum value, thereby to substantially cancel current through said coil means when the current through said positive temperature coefficient resistor means attains said predetermined minimum value.
 2. The combination according to claim 1 wherein the terminal of the secondary winding is connected to a point of reference potential.
 3. The combination according to claim 1 wherein the secondary winding of said transformer means is further coupled with the filament means of the cathode ray tube to provide filament current thereto. 